Last summer (30-07-1997) only one sinistral specimen of Valvata piscinalis was found in the Kanjelbeek (Zuid Limburg) between a lot of dextral ones. Simple genetics as well as the reproduction of Valvata are briefly discussed. The (open) question is raised whether the exceptional appearance of a sinistral Valvata is simply due to genetics together with environmental circumstances or that more complex factors are involved.